A nonaqueous electrolyte battery which is charged and discharged by movement of lithium ions between negative and positive electrodes has been actively studied as a high energy density battery.
The nonaqueous electrolyte battery is expected to be utilized also as medium or large size power sources as well as power sources for miniature electronic devices. Cycle life characteristics and high stability are required in such medium or large size applications.
As a positive electrode active material of a nonaqueous electrolyte battery, a lithium transition metal composite oxide is used, for example. As a transition metal, Co, Mn, Ni, or the like is used. As inexpensive and highly stable positive electrode materials, spinel type lithium manganate, olivine type compounds such as olivine type lithium iron phosphate, and olivine type lithium manganese phosphate have been actively studied in recent years.
Among these, since the olivine type compounds have a low electron conductivity, it has been difficult to obtain good rate performance. In particular, it has been difficult to obtain a lithium manganese phosphate having good charge/discharge properties. Heretofore, carbon coating for enhancing electron conductivity has been known. Further, it has been known that electron conductivity is enhanced by increasing the amount of carbon coating applied to an olivine type compound to enhance the charge/discharge properties. Furthermore, it has been found that deterioration of lithium manganese phosphate progresses due to reduction in the potential at the end of discharge.